1. Field of the Invention
The present invention is directed to a fuel delivery system for an internal combustion engine, and more particularly to a method and apparatus for improving the cold starting characteristics of an internal combustion engine having a diaphragm carburetor.
2. Description of the Related Art
Hand held power devices such as chainsaws, hedge trimmers, line trimmers and edgers are often powered by small internal combustion engines outfitted with diaphragm carburetors. Generally, a diaphragm carburetor has an air passage with a venturi, a diaphragm pump, a needle valve and a metering chamber containing a spring biased diaphragm. The outlet of the air passage leads to the crankcase of the engine. A throttle valve of the butterfly type is typically mounted in the air passage to control the amount of fuel and air entering the crankcase.
Fuel is drawn into the carburetor by the diaphragm pump, which is connected to the metering chamber through the needle valve. The metering chamber, in turn, is connected to the air passage through supply passages fitted with one-way valves. The supply passages open to the air passage through a plurality of outlet ports. The opening and closing of the needle valve and, thus, the flow of fuel into the metering chamber is controlled by a spring biased diaphragm, which is mounted inside the metering chamber.
During normal operation of the engine, pulses of pressure from the engine cause the diaphragm pump to pump fuel from a storage tank up to the needle valve. Subatmospheric air pulses passing through the venturi create a negative pressure in the metering chamber, causing a displacement of the metering chamber diaphragm. The displacement of the diaphragm opens the needle valve and permits fuel to enter the metering chamber. The fuel exits the metering chamber through the outlet ports and enters the air passage where it is atomized. Eventually, the flow of fuel into the metering chamber increases the pressure in the metering chamber, causing the diaphragm to close the needle valve and stop the flow of fuel. As the fuel empties from the metering chamber, the pressure in the metering chamber drops until the diaphragm is again displaced and the needle valve opens. In this manner, the diaphragm in the metering chamber continually opens and closes the needle valve, thereby introducing metered amounts of fuel into the air passage.
Since the delivery of fuel in a diaphragm carburetor is not dependent upon gravity, the operation of a diaphragm carburetor is not affected by its spatial orientation. Accordingly, diaphragm carburetors are ideally suited for use in power devices such as chainsaws that may be held by an operator in a variety of positions. Engines utilizing diaphragm carburetors, however, tend to be difficult to start after a period of non-use because of an initial absence of fuel in the metering chamber and the diaphragm pump. Air choke mechanisms are utilized to remedy this situation. However, most air choke mechanisms are unable to quickly and efficiently establish a proper air to fuel ratio and can flood the engine by introducing excess fuel into the engine.
Air choke mechanisms are usually comprised of slide valves or butterfly valves. Typically, a butterfly valve will be rotatably mounted inside the air passage near the inlet. The butterfly valve often has a small orifice passing therethrough. Usually, the butterfly valve can be rotated between three different positions: an open position, a half-choke position and a full choke position. When the butterfly valve is in the open position, the inlet to the air passage is substantially open. In the half-choke position, the butterfly valve is partially closed and, thus, partially blocks the inlet to the air passage. In the full-choke position, the butterfly valve is closed and blocks the inlet to the air passage except for the small orifice. When the engine is cranked during starting, by a pull rope or otherwise, air is drawn out of the air passage and into the engine. If the choke mechanism is in a full-choke position or a half-choke position, the withdrawal of air creates a negative pressure condition in the air passage. Of course, the amount of pressure reduction is greater in the full-choke position than in the half-choke position. The negative pressure in the air passage creates a negative pressure in the metering chamber which displaces the diaphragm and allows fuel to enter the metering chamber and thence the air passage, where it mixes with air to create an air/fuel mixture.
During the initial cranking cycle, the choke mechanism is placed in a full-choke position to create a maximum vacuum in the air passage. In addition, the throttle valve is fully opened to permit the maximum vacuum to be applied to the outlet ports so as to create a maximum fuel draw. The opening of the throttle valve also permits a maximum amount of the air/fuel mixture to reach the crankcase of the engine. In the full-choke position, however, the air/fuel mixture is very fuel-rich since only a small quantity of air can enter the air passage through the choke mechanism. As the engine begins to fire, more air is required to provide an adequate air/fuel ratio to keep the engine running. Accordingly, the choke mechanism must be moved to the half-choke position as soon as the first internal explosion, or "pop" occurs in the engine. If the choke mechanism is left in the full-choke position for too many cranking cycles after the "pop" occurs, the engine will become flooded with fuel and will not start. The engine will have to be allowed to rest long enough to permit the excess fuel in the crankcase and/or the combustion chamber to evaporate and a proper fuel-air mixture to be restored.
In the half-choke position, the choke mechanism increases the air content in the air/fuel mixture, but still provides a rich-running condition required by the engine during warm-up. After the engine has been running for a few seconds, the choke mechanism must be moved from the half-choke position to the open position to provide a correct air/fuel ratio.
As can be appreciated, the foregoing starting procedure is cumbersome and requires a skilled operator. Accordingly, a variety of priming systems have been developed to help improve the starting characteristics of internal combustion engines with diaphragm carburetors. The object of these priming systems is to introduce fuel into the air passage as soon as the engine cranking cycles are started. One example of a priming system is the air purge system disclosed in U.S. Pat. No. 4,271,093 to Kobayashi, incorporated herein by reference. In Kobayashi, a manually operable resilient pressure dome is connected to the metering chamber and an opening to the atmosphere. When the pressure dome is repeatedly depressed, air from the metering chamber is pulled into the pressure dome and expelled through the atmospheric opening, thereby creating a subatmospheric pressure in the metering chamber. The negative pressure opens the needle valve, partially filling the metering chamber with fuel. When the engine cranking cycles begin, the fuel in the metering chamber is pulled into the air passage through the outlet ports. The amount of fuel in the metering chamber, however, is often insufficient to start the engine, necessitating further engine cranking cycles with the air choke mechanism at a full-choke position. Thus, the Kobayashi system does not eliminate the full-choke and half-choke starting procedure.
In a priming system disclosed in U.S. Pat. No. 4,936,267 to Gerhardy, incorporated herein by reference, the diaphragm in the metering chamber is mechanically deflected by a push rod prior to starting. A positioning lever is connected to both the push rod and a throttle valve. Prior to starting, the positioning lever is pivoted so as to simultaneously move the throttle and depress the push rod. The depression of the push rod deflects the diaphragm and opens the needle valve, permitting fuel to enter the metering chamber. The fuel exits the metering chamber through channels that open into the air passage. Since fuel continues to flow into the metering chamber and air passage until the push rod is manually released, the Gerhardy system is conducive to flooding.
In U.S. Pat. No. 4,508,068 to Tuggle, incorporated herein by reference, a priming system is disclosed wherein fuel is injected directly into the air passage. In addition to a metering chamber, the Tuggle system has a reservoir chamber with a flexible diaphragm wall. The reservoir chamber has an inlet connected to a fuel line leading to a fuel tank with a manually operated plunger pump. An outlet in the reservoir chamber is connected to a flow restricting orifice that opens into an intake manifold portion of the engine downstream of the air passage and the throttling valve. When the plunger pump is depressed, fuel is drawn from the fuel tank and pumped into the reservoir chamber through the fuel line. When the engine cranking cycles begin, the fuel in the reservoir chamber is pulled into the manifold through the restricting orifice. This operation of the Tuggle system is also conducive to flooding because the plunger pump can be depressed too many times, forcing an excessive amount of fuel out of the reservoir chamber and into the manifold.
In U.S. Pat. No. 4,893,593 to Sejimo et al, incorporated herein by reference, a direct fuel introduction system is disclosed for an internal combustion engine having an electric starter motor. In addition to having a metering chamber and other conventional diaphragm carburetor components, the Sejimo system includes a primer pump coupled to the electric starter motor, a fuel reservoir and a fuel metering device, which is separate and distinct from the metering chamber. Before the engine is started, the starter motor and, thus, the primer pump are placed into reverse. When the primer pump is reversed, a negative pressure is created in the metering chamber, causing the needle valve to open and emit fuel into the metering chamber. Fuel exits the metering chamber, fills part of the fuel metering device and then continues into the fuel reservoir. When the starter motor and, thus, the primer pump are placed into forward during starting, the primer pump draws fuel from the fuel reservoir and pumps it into the filled chamber of the metering device, causing the fuel contained therein to be ejected into the air passage.
As can be appreciated, the foregoing prior art priming systems have various drawbacks. The Kobayashi system does not eliminate the need for a full-choke/half-choke starting procedure. The Tuggle system and the Gerhardy system are conducive to over-priming, which can lead to engine flooding. The Sejimo system can only be used with engines having electric starters. Accordingly, there is a need in the art for a fuel delivery system that can quickly start an internal combustion engine without requiring the use of an electric starter motor and without being susceptible to over-priming. In addition, and more specifically, there is a need in the art for a carburetor that can quickly start an internal combustion engine without being susceptible to over-priming and without requiring an electric starter motor. There is also a need in the art to have a method for preparing an internal combustion engine for starting and a method for starting an internal combustion engine that do not require the use of an electric starter motor and are not susceptible to over-priming. The present invention is directed to such a system and to such a carburetor and to such methods.